1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a system and method for controlling power factor in a switching power converter operating in discontinuous conduction mode.
2. Description of the Related Art
Various standards bodies establish consumer product energy efficiency standards, such as power quality. Power Factor (PF) is one measure of power quality and represents a measure of how efficiently energy is drawn from an alternating current (AC) source. For example, the Energy Start 80 Plus Platinum standard specifies that products must exceed a power factor of 0.9 from 50% to 100% of maximum power load. Power supply designers often use active power factor correction (PFC) circuits to meet the PF requirements.
FIG. 1 depicts a boost-type switching power converter, which converts an input voltage from an AC voltage source into a boosted output voltage supplied of the load. The AC input voltage passes through an optional electro-magnetic interference (EMI) filter and then a rectifier. The rectified AC voltage is the input to the boost converter which includes the input and output capacitors (respectively, Cin and Clink), the complimentary switches (Q1 and D1) and the inductor (LBoost). The output voltage of the boost converter, VLink, is a DC regulated voltage that is commonly used as the input voltage to an isolated DC-DC converter stage.
The key principle that drives the boost converter is the tendency of an inductor to resist changes in current. When being charged, the inductor LBoost accumulates energy, when being discharged the inductor LBoost transfers the accumulated energy acting like a source. The voltage produced by the inductor LBoost during the discharge phase is related to the rate of change of current and not to the original charging voltage, thus allowing different input and output voltages.
FET Q1 is driven by a pulse width modulated signal, having a frequency of FSW, applied at the gate of the FET Q1. In a charging phase, the FET Q1 is ON, resulting in an increase in the inductor current (di=v/L·dt). In the discharging phase, the FET Q1 is OFF and the only path for the inductor current is through the fly-back diode D1, the capacitor Clink and the load (DC-DC converter), which results in transferring the energy accumulated by the inductor LBoost during the charging phase into the output capacitor Clink. The input current is the same as the inductor current.
FIG. 2 depicts representative waveforms associated with two different operating modes for the boost converter depending on the inductor current shape. If the current through the inductor LBoost at the end of the discharging phase does not fall to zero, the boost converter operates in continuous mode (CCM); otherwise, the boost converter operates is discontinuous conduction mode (DCM).
The respective CCM and DCM control techniques have their advantages and disadvantages, however, for low power applications, less than 200-300 watts, DCM offers significant performances and efficiency benefits with simpler control algorithms.